Ultrasonography of the gastrointestinal tract: a myriad of disease (Proceedings)
Normal gastrointestinal ultrasound
The patient should be fasted prior to ultrasonography if possible to decrease the amount of gas and ingesta. A high frequency transducer is important (> 7.5 MHz) to maximize resolution and evaluate wall layering. A complete exam of the abdomen is recommended to assess for concurrent disease such as mesenteric lymphadenopathy, pancreatic disease, carcinomatosis, etc. Abdominal radiographs are complimentary to abdominal ultrasound.
Complete ultrasonographic examination of the gastrointestinal tract includes evaluation of wall thickness (from inner mucosal margin to outer serosal margin) and layering, evaluation of luminal contents and determination of peristaltic function. The gastrointestinal tract should be scanned in both the sagittal and transverse planes. For the stomach and duodenum this can be accomplished by scanning the cranial and right abdomen. The colon can also be scanned based on anatomic location. For the small intestines it is best to examine the entire central abdomen in a zigzag pattern. The appearance of the gastrointestinal tract will vary greatly depending on the degree of distention and the luminal contents.When the stomach is empty it will look like a "flower" especially in the cat. In the normal dog the gastric wall is less than 5 mm in thickness and in the cat less than 3.5 mm. These thickness measurements are taken in between rugal folds. Gastric rugae can be recognized in the fundus and body of the stomach with the visibility and thickness dependent on the degree of gastric distension.
The normal thickness of the jejunum in dogs is less than 5 mm and in cats less than 2.5 mm. The duodenum tends to be the thickest area in dogs measuring up to 6 mm. In cats the ileum can measure up to 3.2 mm and has a prominent bright submucosa. The cat ileum has a distinctive "spoke-wheel" appearance. The large intestine is the thinnest and is usually less than 1.5 mm, but can be up to 3 mm in the dog and 2.5 mm in the cat if non-distended.
Ultrasonography allows for differentiation of the layers of the gastrointestinal tract, which alternate in echogenicity. Under optimal conditions, five separate layers can be identified. These include the luminal-mucosal interface (hyperechoic), mucosa (hypoechoic), submucosa (hyperechoic), muscularis (hypoechoic), and subserosa-serosa (hyperechoic). The submucosa and subserosa-serosa are hyperechoic because of the presence of relatively more fibrous connective tissue. The mean number of peristaltic contractions in the gastrointestinal tract is 4-5 per minute.
The ultrasonographic appearance of the lumen depends on its contents. In a collapsed stated the bowel lumen appears as a hyperechoic core ("mucosal stripe") surrounded by a hypoechoic halo of bowel wall. This hyperechoic core represents mucus and small air bubbles trapped at the mucosal-luminal interface. When fluid is present in the bowel lumen, an anechoic area is present between the walls of the bowel that appears tubular in long axis views, and circular in short axis views. Gas within the lumen of the intestine results in a highly echogenic interface with a distal acoustic shadow. The shadow most often is filled with multiple echoes caused by reverberation ("dirty shadow"), but it may also be anechoic ("clean shadow"). The presence of fluid in the bowel lumen improves the sonographer's ability to evaluate the mucosal and submucosal layers of the GI tract, whereas the presence of gas hinders it.
Abnormal gastrointestinal sonographic findings
Lymphosarcoma is the most common type of feline gastrointestinal neoplasm and occurs in the dog as well. The most common ultrasonographic features of lymphosarcoma are thickening of the stomach or bowel wall, loss of its normal layered appearance with reduced echogenicity of the wall, decreased motility, and lymphadenopathy. Diffuse disease can also occur with lymphoma.
Carcinomas are the most common gastric neoplasia in the dog. These usually originate in the pylorus, but may occur in any location in the stomach and also within the intestine. If the mass lesion is in the outflow region (pylorus) the stomach may appear severely distended with fluid, fluid and gas or empty post vomiting. Focal changes are often seen with carcinoma. Wall thickening is more often asymmetric, but it can be symmetric. The loss of the normal layered appearance of the gastrointestinal wall reflects infiltration of neoplastic and inflammatory cells, necrosis, edema, and hemorrhage. Carcinomas have been described as having a pseudolayered appearance and more commonly result in mechanical ileus when compared to lymphoma.
Gastrointestinal leiomyosarcomas can be incidental findings during routine abdominal ultrasound examination. They are usually identified as fairly small round masses protruding into the gastric lumen in the area of the cardia if they are within the stomach. In the intestinal tract they are often large, eccentric, heterogenous masses. In many animals gastric leiomyomas are not associated with clinical signs.